Disk IO is not something that parallelises well (except maybe on SSD) - and this is especially true on optical drives, which have low (compared to HDD - tiny compared to SSD) bandwidth, and pretty huge seek times. If you start seeking with multiple threads at once... yeah, that will be really slow.
Try to avoid having more than one thread accessing the disk. What you could do is have one thread to read data, which then pushes work into a work queue - and then as many workers as you need waiting to take work from the queue and process it. This will minimize IO contention, while still allowing the actual work to be processed in parallel.